Eye based biometric authentication apparatuses rely on acquiring images of a subject's eye under infrared or near infrared illumination.
FIG. 1A illustrates an eye based biometric system 100 comprising an imaging apparatus 102 for acquiring images of a subject's eye coupled with an image processing apparatus 104 for extracting biometric information from acquired images, and optionally analysing such information for the purpose of biometric authentication.
FIG. 1B illustrates an imaging apparatus of the kind more generally illustrated in FIG. 1A, comprising an illuminator IL and imaging camera IC respectively configured such that illuminating radiations from illuminator IL are scattered off a subject's eye E onto imaging camera IC for image acquisition purposes. In embodiments related to eye based biometric authentication (and particularly iris based biometric authentication), illuminator IL emits near infrared wavelengths in the range between 700 nm and 1000 nm.
Since eye based biometric authentication systems rely on infrared or near infrared illumination, image sensors configured to enable such biometric authentication require sensitivity to infrared or near infrared illumination. For the purposes of normal photography on the other hand, infrared or near infrared wavelengths are typically sought to be filtered out. Devices which require to be configured for both normal photography and infrared or near infrared based biometric authentication therefore use two imaging cameras, one configured for infrared or near infrared sensitivity and the other configured for color sensitivity (and infrared or near infrared insensitivity).
The imaging camera configured for infrared or near infrared sensitivity may comprise an image sensor 202 of the kind illustrated in FIG. 2A, having an array of pixels that are sensitive to the desired infrared or near infrared wavelengths. Conventionally, such pixels have also been sensitive to visible wavelengths within the visible spectrums. The imaging camera configured for color sensitivity (and preferably infrared or near infrared insensitivity) may comprise an image sensor 204 of the kind illustrated in FIG. 2B, having a pixel array wherein each pixel is sensitive to one or more wavelengths within the visible spectrum (e.g. red (R), green (G) or blue (B) wavelengths). Each of these pixels may have an infrared or near infrared mask or filter disposed therein, to ensure insensitivity or low sensitivity to infrared or near infrared wavelengths.
FIGS. 2C and 2D illustrate typical prior art RGBIR pixel arrays of the kind used for infrared wavelength based biometric recognition.
In the array 206 shown in FIG. 2C every pixel is sensitive to IR, and is also sensitive one of red (R), blue (B) or green (G) wavelengths. In imaging conditions where the intensity of infrared wavelengths incident upon the image sensor is strong (e.g. outdoors), the infrared wavelengths interfere with accurate reproduction of visible colors in the acquired image. The resolution in the infrared spectrum or near infrared spectrum exhibited by the pixel array of FIG. 2C is good, since every pixel is sensitive to infrared. However, due to the high sensitivity of pixels to infrared wavelengths, the illustrated pixel array has limited effectiveness for visible color imaging outside of a narrow range of ambient light conditions—which limits its effectiveness for eye based recognition to a narrow set of working conditions.
The pixel array 208 illustrated in FIG. 2D addresses the issue of visible color reproduction in environments where ambient infrared wavelengths are strong. However, the image resolution in the infrared wavelength component is reduced to ¼ in comparison with image resolution in the infrared wavelength component exhibited by the pixel array of FIG. 2C. This reduction makes it difficult for the pixel array of FIG. 2D to achieve accurate eye-based biometric recognition within a fixed set of image sensor size and cost constraints.
It is common to use RGB sensors in conjunction with an external infrared cut filter, in order to block infrared light from negatively affecting color reproduction. Similarly it is common to use RGBIR sensors in conjunction with an external dual-band-pass filter, where one band passes visible light and the second passes only the desired portion of the infrared spectrum. It is understood that spectral sensitivity of the pixels refers to sensitivity only to the portion of the radiation spectrum that is not blocked by the external optical filter.
There has additionally been an increasing demand for dual use cameras that can be used for eye based biometric imaging and also for non-biometric imaging purposes (such as regular photography or videography). This need has been particularly felt in the case of mobile communications devices and mobile computing devices, where size, weight and power consumption, place critical limitations on the number of components that can be incorporated within a single device.
There is accordingly a need for technology that advances performance of image sensors having a pixel array that simultaneously includes pixels respectively sensitive to visible wavelengths and pixels sensitive to infrared or near infrared wavelengths.